Engine manufacturers have focused on improving engine design in order to minimise emissions of particulate emissions, emissions of other pollutants, cleanliness and also improve fuel economy and efficiency. One of the improvements in engine design is the use of exhaust gas recirculation (EGR) engines. Whilst improvements in engine design and operation have contributed, improved formulation of engine oil lubricants may also increase cleanliness and minimise engine deposits which accumulate when the engines are running. However, a number of engine design enhancements have led to increased formation and/or accumulation of soot and sludge.
Increased soot thickening is common in heavy duty diesel engines. Some diesel engines employ EGR. The soot formed in an EGR engine has different structures and causes increased viscosity of engine lubricant at lower soots levels than formation of soot in the engine without an EGR. Attempts to alleviate soot formation are disclosed in the references summarised below.
Traditional dispersant viscosity modifiers (DVMs) made from ethylene-propylene copolymers that have been radically grafted with maleic anhydride and reacted with various amines have shown desirable performance to prevent oil thickening in diesel engines. Aromatic amines are said to show good performance in this regard. DVMs of this type are disclosed in, for instance, U.S. Pat. Nos. 4,863,623; 6,107,257; 6,107,258; and 6,117,825.
U.S. Pat. No. 4,863,623 discloses controlling EGR soot by utilising maleic anhydride grafted ethylene-propylene copolymers capped with aromatic amines, such as 4-aminodiphenylamine.
U.S. Pat. No. 5,409,623 discloses functionalised graft copolymers as viscosity index improvers, containing an ethylene alpha-monoolefin copolymer grafted with an ethylenically unsaturated carboxylic acid material and derivatised with an azo-containing aromatic amine compound.
U.S. Pat. No. 5,356,999 discloses multifunctional viscosity index improvers for lubricating oils containing a polymer onto which has been grafted an unsaturated reactive monomer and thereafter reacted with amines containing sulphonamide units. The polymer is either an ethylene-propylene copolymer or an ethylene-propylene-diene terpolymer.
U.S. Pat. No. 5,264,140 discloses an ethylene alpha-monoolefin copolymer grafted with an ethylenically unsaturated carboxylic acid derivatised with an amide-containing aromatic amine material.
International publication WO 06/015130 discloses maleic anhydride grafted ethylene-propylene copolymers capped with sulphonamides, nitroanilines, diaromatic diazocompounds, anilides or phenoxyanilides. The copolymers are useful for controlling EGR soot.
Other dispersant viscosity modifying polymers suitable for lubricants have been contemplated including polyacrylic copolymers, including the disclosure of British Patent GB 768 701.
U.S. Pat. No. 4,234,435 discloses a composition in which a succinated polybutene is condensed with either an alkyl polyamine to make a succinimide dispersant or an alkyl polyol to make a succinic ester dispersant.